In the vastness of the universe, collisions between galaxies are surprisingly common. In this particular cosmic train wreck between two galaxies located about 38 million light-years away, the result apparently was the creation of a gigantic circle of gas. For nearly 30 years, astronomers have wondered what formed the so-called Leo ring—named because the galaxies are located within the constellation Leo. Observations in infrared light had suggested the ring was composed of cold primordial gas left over from the big bang. But a new study, to be reported in The Astrophysical Journal Letters, used visible light. In the images astronomers found evidence of starmaking within the ring—something not possible in primordial gas clouds. Then a computer simulation based on the new observations solved the mystery: about 1 billion years ago, a violent encounter between the two galaxies ripped out their starmaking gas and spread it as a ring some 650,000 light-years wide.